NH2-AA1-AA2-AA3-AA4-AA5-COOH Primary Structure

Definition: Local folded structures formed by hydrogen bonds between the backbone atoms (carbonyl oxygen and amide hydrogen) of the polypeptide chain.
Common types:
- $\alpha$-Helix:
  - Right-handed spiral structure.
  - H-bonds form between $C=O$ of residue $n$ and $N-H$ of residue $n+4$.
  - Side chains point outwards.
- $\beta$-Pleated Sheet:
  - Polypeptide chains lie side-by-side.
  - H-bonds form between adjacent strands.
  - Can be parallel or anti-parallel.
- $\beta$-Turns (or Reverse Turns):
  - Connect strands of anti-parallel $\beta$-sheets.
  - Involve 4 amino acid residues.

Definition: The overall three-dimensional shape of a single polypeptide chain, including the spatial arrangement of its secondary structures and side chains.
Stabilized by various interactions between amino acid side chains:
- Hydrophobic interactions (nonpolar side chains cluster in the interior).
- Ionic bonds (salt bridges between charged side chains).
- Hydrogen bonds (between polar side chains).
- Disulfide bridges (covalent bonds between two cysteine residues).
- Van der Waals forces.
Critical for protein function.

Definition: The arrangement of multiple polypeptide chains (subunits) in a multi-subunit protein complex.
Not all proteins have quaternary structure (only those with multiple subunits).
Subunits can be identical (homo-oligomer) or different (hetero-oligomer).
Stabilized by the same non-cova